After a severe accident in nuclear power plants occurs, it is important to grasp a state inside a pressure vessel and a containment vessel so that appropriate measures are taken to control the accident.
Conventionally, the nuclear power plants include a measuring system assumed to be used not only in monitoring at the time of normal operation but also in case of an accident. For example, the measuring system is used for sampling a hydrogen and/or oxygen concentration, sampling radioactivity, and monitoring radiation in a containment vessel, as well as monitoring a fuel damage situation and emission of hydrogen and radioactive substances from the damaged fuel (see, for example, Patent Documents 1 to 3).
To prevent fuel damage, monitoring of a water level is important. For example, boiling water reactors (BWRs) use a differential pressure type level gauge to monitor a water level during normal operation as well as to monitor a fall of the water level to a fuel area (see, for example, Patent Document 4). The differential pressure type level gauge is also used for water level measurement in a suppression chamber which contains cooling water for use in cooling a fuel and the like (see, for example, Patent Document 5).
Ever since the accident in TMI-2 (the Three Mile Island Unit 2) reactor, it has been pursued to reinforce the measuring system to grasp a reactor core cooling situation. For example, pressurized water reactors may have a thermocouple water level monitor attached to an upper portion of a pressure vessel (see, for example, Patent Document 6). Further, ever since the Chernobyl accident in Europe, diversified monitoring methods have been demanded and thereby review of safety measures have been conducted.
Diversification of the water level monitoring is also demanded. For example, water level monitoring by using ultrasonic waves, microwaves, thermocouples, and/or gamma rays outside the reactor is being examined (see, for example, Patent Documents 7 to 10).
In addition, for monitoring the state inside the containment vessel, measurement of flow rates, pressures, air temperatures (ambient temperatures) at various positions, and/or surface temperatures of devices are being performed.